It is well known in the pulp, paper or board making industry to apply oxidizing or non-oxidizing biocides to control microbial growth. Examples of commonly used non-oxidizing biocides are glutaraldehyde, 2,2-dibromo-3-nitrilopropionamide (DBNPA), 2-bromo-2-nitropropane-1,3-diol (Bronopol), quaternary ammonium compounds, carbamates, 5-chloro-2-methyl-4-isothiazolin-3-one (CMIT), and 2-methyl-4-isothiazolin-3-one (MIT). Typical examples of commonly used oxidizing biocides are chlorine, hypochlorite salts, hypochlorous acid, chlorinated isocyanurates, bromine, hypobromite salts, hypobromous acid, bromine chloride, chlorine dioxide, ozone, hydrogen peroxide, or peroxy compounds.
A specific application of biocides is the control of starch degradation in process waters of the paper industry. Starch is a widely used additive in paper making. Actually paper making is the largest non-food usage of starch. For example, in the wet end of a paper machine, starch is used to improve paper strength. In the dry end of a paper machine, starch is used for coating the paper in a process called surface sizing. This gives paper additional strength and better printing properties. Amylase is an enzyme that catalyzes degradation of starch. It is produced by many microorganisms, both fungi and bacteria, and is also present for example in human saliva. Amylase enzymes are divided into three groups: α-, β- and γ-amylases. They all hydrolyse α-1,4-glycosidic bonds that link together glucose units of starch molecule. β-amylase can break only the second α-1,4-glycosidic bond, yielding into two glucose units (maltose). α-amylase can attack any bonds in the starch molecule and thus is often faster acting than β-amylase. γ-amylase cleaves one glucose unit at the time and is most efficient in acidic environments.
Process waters in the paper industry can contain microorganisms which can produce amylase enzymes that degrade starch and cause loss of the functionality of added starch additive. This will lead either to paper quality issues, or alternatively force to increase starch dosages thus creating unwanted additional costs.
Current practices in controlling starch degradation have been inadequate in efficacy or have required economically unfeasible high biocide dosages. Especially when process water with high amylase activity is used for pulping of recycled fiber, or in re-pulping of dry broke of a paper or board machine, degradation of starch is easily taking place and the benefits of the starch already included in the fibrous material from recycled paper (which contains plenty of starch from the original production process) is lost.
WO 2012/025228 A1 discloses a method for manufacturing paper, wherein a cellulosic material containing starch is treated with biocides, followed by adding an ionic polymer and an auxiliary ionic polymer, both polymers having different average molecular weight and different ionicity.